Data recording media such as optical disks, hard disks, etc., are capable of recording large quantities of information. Such data recording media are commonly referred to as CD's (compact disks), LD's (laser disks), DVD's (digital video disks, digital versatile disks), etc. These data recording media may contain music, movies, software, etc. Such media are also used as storage devices in computers. Demand for such recording media is expanding greatly. Indeed, it is anticipated that optical disk and hard disk usage will continue to expand since these are the major recording media of the multimedia age.
Optical disks are classified according to the existence or absence of a recording layer and further classified according to the type of recording layer. Optical disk types include: (1) the read-only type (CD, LD, CD-ROM, photo-CD, DVD-ROM, read-only type MD, etc.); (2) the write-once type (CD-R, DVD-R, DVD-WO, etc.); and (3) the rewriteable type capable of erasure followed by writing any number of times (magneto-optical disk, phase-change type disk, MD, CD-E, DVD-RAM, DVD-RW, etc.). Moreover, the high density HD-DVD has also been proposed as a medium of the future.
The process for manufacturing these optical disks begins with the molding of raw material resin into a resin substrate. Raw material resin, for example, polycarbonate, acrylate resin, polystyrene, etc., is heated, melted or partially melted, and then pressed using a stamper, thereby molding (manufacturing) a resin substrate. Typically, the molding method used is a pressure molding or injection molding method. The stamper forms fine concavities and protuberances which represent the information to be copied upon the substrate surface. Other than resin molding, there is no such method for manufacturing large quantities of substrates that have minute concavities and protuberances in a short time period.
Types of pits and protuberances include pits that indicate a unit of information and guide grooves that are provided for tracking by the pickup head. Generally, the manufacture of data recording media involves circular substrates provided with pits and grooves on the substrate surface in a pattern of concentric circular rings or as a spiral pattern. The region between grooves along the radial direction is called a "land." Recording upon the lands occurs during the land recording method, or alternatively, recording occurs within the groove per the groove recording method.
In order to improve the recording density, the land/groove recording method was developed to record upon both the grooves and lands. In this case, both grooves and lands are tracks, and the width of both grooves and lands are nearly equal. However, there are reasons for sometimes deliberately widening one or the other. For example, when incident light enters the backside surface (flat smooth surface) of the substrate, what appears as a land from the substrate interior side becomes a groove as seen from the substrate front.
As the recording density has increased, to meet the increased need for storage capacity, the width of grooves, lands, and pits has decreased and their depth has increased. For example, the width has decreased from &lt;1 .mu.m to &lt;0.3 .mu.m and the depth has increased from &gt;40 nm to &gt;250 nm. As the width decreases and the depth increases (i.e., as density become higher), molding of the resin substrate becomes increasingly difficult, and the yield of good product declines.
When manufacturing a hard disk, a magnetic recording layer is typically formed or deposited on an aluminum or glass substrate with recording carried out by a magnetic head. A reflection layer, a recording layer and a protection layer may then be formed on the resin substrate to produce the desired final product.
As recording density increases, the recording layer becomes extremely flat and smooth. When the magnetic head becomes relatively still, the recording head and the recording layer adhere to one other and then fail to separate. In order to avoid this phenomenon, a garage region (CSS region=contact stop and start) is provided. The surface of this garage region is deliberately finished with a rough texture using a laser such that surface adherence is prevented. Head tracking also becomes difficult as recording density increases. Therefore it is proposed that a magnetic hard disk, like an optical disk, should be provided with grooves. Due to the demand for such roughness and grooves, resin substrates are proposed as a means to increase manufacturing productivity. Increased productivity results due to the foundation of roughness and grooves during the substrate molding. Resin substrates are also said to be advantageous due to their light weight.
Previously, molding tools were manufactured by the process described in Hunyar, U.S. Pat. No. 4,211,617, which corresponds to Japanese Publication Koukoku Sho 59-16332, the disclosures of which are incorporated by reference herein in their entirety. A comparison of the method of forming the molding tool as disclosed in Hunyar and that of the present invention is provided in FIG. 3A (present invention) and 3B (Hunyar).
Generally, molding tools are manufactured using a glass substrate that is polished with the precision of an optical surface. After the substrate 1 is cleaned it is coated with a primer, for example, a silane coupling agent. A photoresist is then applied by spin coating and subjected to a pre-bake process. Positive-type, i.e., wherein the region exposed to light is removed during development, photoresist 2 is often used (see item (B1) in FIG. 3). Next, a laser beam recorder or a laser cutting machine is used to expose the photoresist 2 with a pattern of pits and/or grooves where the width of pits and grooves is generally determined by the laser beam diameter and the depth of the pits and grooves is generally determined by the photoresist thickness. When the exposed photoresist is developed, a resist pattern of pits and/or grooves is obtained upon the substrate surface. Following development, the resist pattern may optionally undergo a 20-60 minute post-bake at 80-120.degree. C. When such a post-bake is used, the resist pattern is then cooled down to room temperature. Roughly 10 hours are required to cool the resist pattern. The resist pattern formed in this manner is referred to as a "master substrate" or a "master" and is indicated by item (B2) in FIG. 3 of the present application. It is also shown in FIG. 4, reference number 46, of Hunyar U.S. Pat. No. 4,211,617. In addition to the fact that these master substrates require long production times, the laser beam recorder or a laser cutting machine used to expose the photoresist is very expensive. As a result, master substrates are expensive and time-consuming to produce.
The master substrate then undergoes metallization treatment to form a conductive layer on the surface. Generally such treatment is carried out by sputtering (dry-type method), or by non-electrolytic plating (wet-type method). Following metallization, a thick plating layer, such as nickel (Ni), is formed upon the master substrate. The result is a first metallic molding tool that has a double layer structure that consists of a conductive layer and the Ni plating layer. This is shown by item (B3) of FIG. 3 of the present application. This first metallic molding tool is referred to herein as the "father," 3. A free father is obtained when the father 3 is peeled from the master substrate resist pattern. This is indicated by item (B4) of FIG. 3. The father is equivalent to mother member 52 in FIG. 6 of Hunyar U.S. Pat. No. 4,211,617. Care must be used during peeling since the father is generally thin, approximately 200-300 .mu.m in thickness. After peeling, the father 3 undergoes solvent treatment to remove resist since a portion of the resist may remain on the father 3. Resist must be removed since the concavities and protuberances on the surface of the father would otherwise be destroyed. Only a single father 3 is obtained from a single master substrate since the resist pattern is damaged during peeling. After the resist is removed, the concavity-protuberance surface is shielded with a protective coat and the backside surface is polished. A central hole is bored in the center of the father 3, and the unused portion of the outside perimeter is cut off. This results in an annulus-shaped father.
The father that is completed in this manner has an extremely accurate pattern of concavities-protuberances. Without additional treatment, the father may be used as an injection mold to make resin substrates for use as DVD-RAM, MD, HD-DVD, and other high density recording media (&lt;0.8 .mu.m groove width) that require an extremely high-precision pattern of concavities-protuberances.
However, the father is very expensive due to the high cost of the master substrate and the fact that only a single father is obtained from a single master substrate. Therefore a duplicate of the father is obtained in the same manner using Ni electrotype duplication. This is shown by item (B5) in FIG. 3. This duplicate molding tool is referred to herein as a "mother," 4b. A free mother is obtained when the mother 4b is peeled from the father 3. This mother 4b is equivalent to sub-master 60 in FIG. 8 of U.S. Pat. No. 4,211,617. Prior to electrotype duplication, the mother 4b undergoes surface treatment (passivation) so that the mother will readily peel away from the father 3. A typical surface treatment uses potassium dichromate solution, potassium permanganate solution, etc. It is not possible to reuse the father indefinitely since the father may become somewhat damaged when the mother is peeled away from the father. Generally, a maximum of 2 or 3 uses is possible. Therefore only 2 or 3 mothers can be obtained from a single father. The mother can also be used as a molding tool for injection molding but the concavities and protuberances of the mother are opposite those of the father.
To further increase the number of copies or to reverse the concavitics-protuberances, the mother 4b may be used in place of a master substrate, and a duplicate of the mother 4b is obtained in the same manner using Ni electrotype duplication. This is shown by item (B7) in FIG. 3. This duplicate of the mother is referred to herein as the "son," 6. A free son is obtained when the son is peeled from the mother 4b. This is shown by item (B8) in FIG. 3. The son 6 is equivalent to the stamper member 70 in FIG. 9 of Hunyar U.S. Pat. No. 4,211,617. In order to readily peel the son 6 from the mother 4b, the mother 4b also undergoes passivation prior to electrotype duplication. Since the mother 4b is also somewhat damaged when the son 6 is peeled away from the mother 4b, it is not possible to reuse the mother indefinitely. Typically, a maximum of 2 or 3 uses is possible. Therefore only 2 or 3 sons can be obtained from a single mother 4b.
Due to the damage of the father and mother molding tools, it is typical to form only about 4-9 sons from a single high-cost father. Typically, the son (or possibly the father or mother) is used during injection molding to mold a large number of resin substrates. These substrates are used for data recording media such as optical disks, hard disks, etc. Roughly 20,0000-30,000 resin substrates can be molded from a single son. However, the son itself becomes damaged and unusable after the molding of more than about 20,000-30,000 substrates. Resin substrate quality would decline if the son were to be used further.
Typically, when an extremely high precision concavity-protuberance pattern is required, the father itself is utilized as the mold. However, this results in high cost (problem A). In order to reduce costs, duplicate sons are used as the mold, but these are still expensive since the number of such duplicates which can be accurately produced is low, e.g., about 4-9. Therefore sons are still expensive (problem B). Additionally, volume production using the prior art methods is also difficult (problem C).
It has also been proposed to reduce prices by manufacturing a large number of mothers from a single father, and to manufacture a large number of sons from each mother. However, when this is done, the resultant mothers and sons are not highly precise (problem D). Consequently numerous identical fathers are required. However, for various reasons, minute differences (problem E) occur between fathers when numerous fathers are manufactured. These minute differences are referred to here as "individuality." Due to such individuality, the injection molding conditions, for example, mold temperature and injection pressure, must be carefully adjusted after a father is replaced. Adjustments must be made to the molding conditions used from mother-to-mother or son-to-son. Individuality is quite problematic for manufacturers using injection molding since substrate productivity declines during the adjustment time period (problem F).
Consequently, there is a need in the art for a process which enables the production of low cost, high volume, high precision molding tools.